There are several devices available claiming to determine the rate of evaporation, such as the Piche evaporimeter, clay atmometer, pan evaporimeter and a wide variety of lysimeters. While all of these devices yield data for the evaporated amount of water in a selected period of time, it is understood that those obtained data are only representative of the devices from which the data were obtained. The main problem is that the common standard for the determination of the state of the air, the psychrometer, is separated from the above listed devices while all of them acquire their own temperatures.
There are also psychrometers with reservoirs, but they do not function as evaporimeters. On the one hand, the reservoirs are not made to register a change in the water supply. On the other hand, the reservoirs are substantially open around the wick and the wet bulb. Because of this, evaporation takes place not just from the web bulb, but also from the entire wick and from the water in the reservoir. This evaporation affects the condition of the air in the close vicinity of the wet bulb and hinders the evaporation from it. This results in the incorrect evaluation of the conditions of the surrounding air. None of these psychrometers allows the evaporation to be related to the well defined surface of that portion of the wick which covers the wet bulb. For the above reasons the rate of evaporation from the unit surface cannot be determined by the use of these psychrometers.
The equations, mainly the different modifications of the Penman equation, are derived to calculate the rate of evaporation by the use of psychrometric and wind data with the assumption that to a certain extent the psychrometric data near the ground already reflect the condition of the ground, including the vegetation on it, as well as the absorptive ability of the air. The calculated results, however, are expected to be checked against some sort of observational data of actual evaporation.